Don't call `diff-refine-hunk' when looking at git's combined diff.

fixes #539
Adds management of SVN externals through magit-svn-mode

Assuming the user has cloned all of their SVN externals into a single
directory which can be specified by magit-svn-externals-dir and then
symlinked in the appropriate places, "x" for "Fetch Externals" is added
to the menu and individually fetches and rebases each git svn repository
for the externals.
simplify code

also reformat comment
Merge branch 'danlamanna/git-svn-externals'
Merge branch 'master' into next

Conflicts:
	magit-svn.el